Windows: Fix `Command::env_clear` so it works
Windows: Fix `Command::env_clear` so it works

Previously, it would error unless at least one new environment variable was added.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust